Electrophotographic marking is a well known and commonly used method of copying or printing documents. Electrophotographic marking is performed by exposing a light image representation of a desired document onto a substantially uniformly charged photoreceptor. In response to that light image the photoreceptor discharges, creating an electrostatic latent image of the desired document on the photoreceptor's surface. Toner particles are then deposited onto that latent image, forming a toner image. That toner image is then transferred from the photoreceptor onto a substrate such as a sheet of paper. The transferred toner image is then fused to the substrate, usually using heat and/or pressure, thereby creating a copy of the desired image. The surface of the photoreceptor is then cleaned of residual developing material and recharged in preparation for the production of another image.
The foregoing broadly describes a prototypical black and white electrophotographic printing machine. Electrophotographic marking can also produce color images by repeating the above process once for each color of toner that is used to make the composite color image. The various color toners can then be transferred onto a substrate in a superimposed registration such that a desired composite color image results. That composite color image can then be fused to make a permanent image.
One way of exposing a photoreceptor is to use a Raster Output Scanner (ROS). A ROS is comprised of a light source (or sources) and a rotating polygon that has a plurality of mirrored facets. The light source radiates a laser beam onto the polygon facets, which reflects the beam onto the photoreceptor so as to produce a light spot. As the polygon rotates the light spot traces lines, referred to as scan lines, on the photoreceptor. Since the photoreceptor itself usually moves, the surface of the photoreceptor is raster scanned by the light spot. During scanning the laser beam is modulated so as to produce a latent image on the photoreceptor.
Numerous printing architectures are available for producing composite color images using ROS technology. Significant to the present invention are ROS-based single pass printers wherein a composite color image is produced in a single pass of the photoreceptor through the machine. Single pass ROS-based printers are advantageous in that they are very fast since a color image is produced during each cycle of the photoreceptor.
While raster output scanning is successful, it is not without problems. One set of problems results from facet imperfections. While each polygon facet is ideally perfectly flat, perfectly parallel to the axis of rotation of the polygon, exactly the same as the other facets, and forms the same angle with its adjacent facets, in practice these ideals are not achieved. When multiple polygons are used the number of facet imperfections increases since there are more facets. The various imperfections result in nonuniform scan lines.
While start of scan detectors enable the adjustment of the scan lines such that the scan line latent images align along one edge, this does little to help other problems resulting from facet imperfections. For example, non-flat facets result in spatial nonuniformity: the spot is not where it should be at a particular time. Additionally, facet angle errors result in scan lines that trace across the photoreceptor at different rates and that end at different times. Color print testing performed at Xerox has proven that facet imperfections result in color defects that are referred to herein as color banding: color image to color image misregistration in the final image. In multiple polygon electrophotographic printers this problem is compounded by polygon to polygon differences as well as by the facet to facet differences within a single polygon. Therefore, a technique of reducing color banding in multiple polygon single pass electrophotographic printers would be beneficial.